This invention relates to medical diagnostic imaging systems and, in particular, to diagnostic imaging systems designed for imaging the breast.
The presently preferred imaging modality for breast disease screening is mammography. During a mammographic examination the breast is firmly held between two compression plates while exposed to radiation. Adequate breast compression is fundamental in mammography for optimal image quality. Compression provides breast immobilization which minimizes exposure time and motion blur, and minimizes breast thickness which minimizes geometric blur and absorbed glandular dose. Compression forces can exceed 40 pounds of force for a large breast, so the breast support table is ideally constructed from a material that is both radiographically transparent and mechanically rigid. Epoxy/carbon-fiber composite materials generally satisfy these characteristics.
When a mammogram indicates a suspicious lesion, the next diagnostic step is often to examine the lesion ultrasonically. For this reason it is convenient for the diagnostic instrument to be capable of performing both mammography and sonography. However, in order to combine mammography and sonography into the same scanning device, the breast support table must also be transparent to high-frequency ultrasound. Unfortunately, carbon/epoxy composites are poorly suited for this purpose, since their rigidity creates a large mismatch in sound speed and bulk acoustic impedance compared to human tissue. This mismatch results in high acoustic transmission losses, focusing aberration that degrades spatial resolution, and ring-down artifacts. Conversely, the known polymers with acoustic properties that match human tissue are soft, flexible rubbers or elastomers which readily deflect under large compression forces and are thus poorly suited to act as compression plates. Previous investigators claimed to rely on the intrinsic stiffness of very thin (25 xcexcmm) polymeric films such as Kapton(copyright) polyimide (DuPont, Circleville, Ohio) to provide compression. In practice, it is doubtful that a 25 xcexcmm film of this (or any known) polymer is sufficiently stiff to resist bending under the large compressive forces needed for screening mammography. Conversely, other investigators have recently studied the use of thick polymer plates (xcx9c6 mm) to find the best compromise between acoustic transmission loss and mechanical stiffness. However, the large speed of sound mismatch in a thick plate requires corrections to the beamformer delays to compensate for focusing errors due to refraction. It is thus desirable to provide an acoustic window for the breast support table which simultaneously provides adequate rigidity while maintaining radiographic and acoustic transparency.
In accordance with the principles of the present invention, a compression plate suitable for providing an acoustic and radiographic window for either sonography, mammography, or both is provided by a thin polymer which is mounted under tension. By choosing the appropriate tensile force, the desired rigidity can be provided by a wide range of polymeric membranes. In accordance with a further aspect of the present invention, the thickness of the polymeric membrane is chosen in accordance with the nominal ultrasonic imaging frequency. By choosing a thickness which is a multiple of the ultrasonic wavelength or a fraction thereof such as xcex/2, the membrane can be made virtually completely transmissive at the nominal imaging frequency. The suitability of a wide range of polymeric membranes for the ultrasonic criteria mean that a material with good radiographic transmissivity can be chosen for the acoustic/radiographic window.